An important advance in aircraft flight safety has been the development of ground proximity warning systems. These warning systems analyze the flight parameters of the aircraft and the terrain surrounding the aircraft. Based on this analysis, these warning systems provide alerts to the flight crew concerning possible inadvertent collisions with terrain or other obstacles. Although these warning systems are quite useful in providing the flight crew with information concerning potential problems with the navigation of the aircraft, the usefulness of these systems must be balanced against problems associated with providing false alerts to the flight crew that may cause the flight crew to ignore alarms from the ground proximity warning system altogether.
For example, during the landing operation of the aircraft, the aircraft will follow a flight path that will eventually intersect the earth at the intended runway on which the aircraft is scheduled to land. In the landing operation, ground proximity warning systems, if not adequately controlled, may generate constant alarms. The constant generation of alarms during landing may be a nuisance due to the added stress and confusion the alarms may impose on the flight crew. Additionally, the nuisance alarms may overshadow other critical alarms in the cockpit. For this reason, some ground proximity warning systems anticipate the landing of the aircraft and disable or desensitize alarms otherwise generated by the warning system within a predetermined range of the airport, such that the ground proximity warning system will not generate nuisance alarms during landing of the aircraft.
Although disabling or desensitizing of alarms generated by the ground proximity warning system during landing eliminates problems associated with the generation of "nuisance" warning alarms, determining when to disable the ground proximity warning system also presents several problems. Specifically, several airports are located in geographic areas that are in close proximity to either natural high elevation terrain such as mountains and/or manmade terrain such as skyscrapers. Premature disablement or desensitization of alarms of the ground proximity warning system may disadvantageously eliminate ground proximity protection from these features near the airport.
However, operating the ground proximity warning system in close proximity to the airport may also cause problems. Specifically, if the ground proximity warning system is operated conservatively and the alarms remain enabled in close proximity to the airport, the ground proximity warning system is more likely to give nuisance alarms, mistaking the aircraft trajectory intersection with the runway as requiring a ground proximity alert. In these instances, the flight crew may become desensitized to the alarm and associate the alarm with the impending landing of the aircraft, instead of the terrain surrounding the airport.
Various ground proximity warning systems have been designed that attempt to detect when the aircraft is entering a landing procedure such that the alarms of the ground proximity warning systems may be disabled or desensitized in a more timely and sophisticated manner. For example, some ground avoidance systems monitor the flaps and landing gear systems of the aircraft to determine if these systems are operating in a characteristic landing configuration. Other systems monitor the rate of descent and air speed of the aircraft to determine whether the aircraft is landing.
Although these systems are designed to determine when the aircraft is beginning a landing procedure, these systems may at times be unreliable, because configurations of the flaps, landing gear, air speed, and rate of descent that may appear to be part of a landing procedure, may also be configurations used in normal flight of the aircraft. Additionally, use of flap and landing gear configurations as indications of landing may cause the ground proximity warning system to not timely disable or desensitize the alarms. Specifically, because the flight crew typically configures the flaps and landing gear, the timing of the configuration of the flaps and landing gear may be different for each landing. Thus, the warning alarms of the ground proximity warning system may either remain enabled for too long and produce unwanted nuisance alarms during a portion of the landing procedure, or the alarms of the ground proximity warning system may be disabled too early and not provide adequate protection from terrain near the airport.
Other ground proximity warning systems have been developed that evaluate the proximity of the aircraft to an airport and the flight altitude of the aircraft above the runway to determine if the aircraft is entering a landing procedure. For example, one ground proximity warning system monitors the altitude of the aircraft in relation to the runway closest to the aircraft. If the aircraft approaches the runway within a predetermined distance range and within a predetermined altitude range, the ground proximity warning system will determine that the aircraft is entering a landing procedure. During the landing procedure, the ground proximity warning system creates a terrain floor surrounding the runway. The generation of the terrain floor is discussed in detail in U.S. Pat. No. 5,839,080, entitled "Terrain Awareness System" which is assigned to the assignee of the present application. The contents of U.S. Pat. No. 5,839,080 are incorporated herein by reference.
As detailed in U.S. Pat. No. 5,839,080, the terrain floor represents minimum altitudes required by the aircraft at certain distances from the runway in order to safely approach the runway according to conventional landing procedures. Additionally, the terrain floor includes an area immediately adjacent to the runway where the alarms of the ground proximity warning system are not generated, such that the ground proximity warning system does not generate nuisance alarms during the final approach of the aircraft to the runway. This ground proximity warning system provides several advantages as it does not require the monitoring of landing gears and flaps, but instead monitors the positional relationship between the airport and the aircraft.
Although the above mentioned ground proximity warning system provides several advantages, there may be instances where the use of the closest runway to the aircraft in the creation of the terrain floor may not provide desired accuracy for the operation of the ground proximity warning system. Specifically, there may be instances where the aircraft approaches the airport from one direction with intentions of landing on a runway on the opposite side of the airport. In these instances, the above-mentioned ground proximity warning system may choose the closest runway to the aircraft as the aircraft approaches the airport and may disable or desensitize the alarms of the ground proximity warning system based on the distance and altitude relationship between the aircraft and the closest runway, instead of the intended landing runway. As such, the ground proximity warning system may prematurely disable or desensitize the alarms, thereby possibly not providing maximum ground proximity warning protection in the area close to the runway where the aircraft is intending to land.
An additional problem may be experienced where two airports at different elevations above sea level are located in close proximity to one another, and an aircraft flies near one airport at low altitude in route to the second airport. In these instances, as the aircraft flies near the first airport located at one elevation above sea level, the ground proximity warning system will use the closest runway of the first airport in the creation of the terrain floor. Based on the distance from the closest runway, the ground proximity warning system will provide certain indications to the flight crew of the aircraft, such as terrain caution and terrain warning alerts and a display that depicts the surrounding terrain that is colored to reflect the aircraft's proximity to the terrain based upon the incorrect assumption that the aircraft is landing at the closet runway at the first airport. However, when the aircraft flies past the first airport in route to land at the second airport, the ground proximity warning system will choose the closest runway at the second airport that is located at a different elevation above sea level than the previous selected runway. The change in elevation between the two different runways used in the ground proximity warning calculations may cause the system to dramatically alter the manner in which the surrounding terrain is colored upon the display so as to confuse and possibly alarm the flight crew. In addition, any terrain caution or terrain warning alerts generated based upon the incorrect assumption that the aircraft was landing at the first airport may very well be erroneous for an aircraft landing at the second airport.